1. Field of the Invention
The present invention relates to a method and apparatus for forming a polycrystalline layer using laser crystallization, and more particularly, to a method and apparatus for forming low temperature polysilicon (LTPS) using sequential lateral solidification (SLS) excimer laser annealing.
2. Description of the Related Art
Thin film transistors are typically employed as active elements for driving active matrix liquid crystal displays (LCDs), active matrix organic light-emitting displays (OLEDs), and the like. The silicon layer in the thin film transistor comprises a polysilicon or amorphous silicon (a-Si:H) layer.
Amorphous silicon has advantages such as lower process temperature, easy mass production by chemical vapor deposition, and high production yield due to mature process technique. On the other hand, polysilicon possesses excellent conductivity and high field effect mobility such that the transistor can be applied in high speed operating circuits, and offers excellent integration of driving circuits. With the development of low temperature processes, polysilicon TFTs can replace amorphous silicon TFTs.
There are three conventional methods for fabricating a polysilicon layer. The first method forms a polysilicon layer using conventional high temperature deposition method. The second method forms an amorphous silicon layer, followed by heating and crystallizing to form a polysilicon layer. The third method forms an amorphous silicon layer, followed by laser irradiating and crystallizing to form a polysilicon layer. However, the first method has the disadvantage of quite thick deposition of polysilicon to form a large grain polysilicon layer and poor uniformity. The second method is capable of growing a thin and uniform polysilicon thin film but suffering slow throughput and low yield resulting from holding for several hours at about 600° C. The third method, such as an excimer laser annealing (ELA) process, has a lower temperature requirement, but undergoes quite slow throughput due to a slow scan speed of 6 mm/sec and irradiation energy of 370 mJ/cm2. The sequential lateral solidification (SLS) method offers a higher scan speed of 300 mm/sec and higher irradiation energy of 600 mJ/cm2, and thus the throughput is raised. FIG. 1 illustrates the thickness distribution of an amorphous layer 14 deposited on a buffer layer 12 over a glass substrate 10. The amorphous silicon layer 14 in the peripheral region A is thinner than that in the main area C. The thinner amorphous silicon layer region (i.e., peripheral region A) may be destroyed due to laser irradiation with slow scan speed. Besides, alignment marks located in the peripheral region A may be also destroyed too. Thus, it may impede the subsequent process.
Accordingly, the sequential lateral solidification (SLS) laser annealing process requires an extended distance for acceleration of the annealing apparatus to a scan speed of 300 mm/sec prior to reaching the peripheral sections. More specifically, the SLS laser needs to start irradiation and gets up to constant speed from outside of the glass substrate. This creates the problem of needs more space outside the substrate.
To prevent the amorphous silicon layer 14 in the peripheral region A from damage by laser irradiation, a line beam laser irradiates the uniform region of the amorphous layer 14 in the main region C. Since the sequential lateral solidification (SLS) laser annealing process irradiates from the outside of the glass substrate, the shutter of the laser annealing apparatus is controlled to be opened only when directed at the uniform region for preventing damage to the peripheral region A of the amorphous layer 14 by laser irradiation. In FIG. 1, the shutter opens at D and E; and is fully open at d and e. The region wherein the shutter is not fully open does not receive enough irradiation energy to crystallize the amorphous layer 14 hence the polycrystalline area of the substrate is reduced.